feat: update merge_sort.ts

[benben6515]

• use JavaScript ES6 syntax
• use let, const instead of var
• make it more "clean", I supposed

[appgurueu]

I support replacing var with let/const. However your "shortening" comes at the cost of efficiency:

• You are copying both the result array and the remaining slices to a new final array.
• You are not allocating the proper size for result anymore.

You also introduced a minor behavioral change: Before, mergeSort would always return a copy. Now it will return the same array if the length is <= 1. That is, it is "in-place" when the problem is trivial, and copies otherwise. E.g. the following would work:

const a = [42];
const b = mergeSort(a);
b[0] = 101;
console.log(a[0]); // 101

but this wouldn't:

const a = [33, 42];
const b = mergeSort(a);
b[0] = 101;
console.log(a[0]); // 33

Also please fix the test by applying the rename there too.

[benben6515]

Sorry about the unconsidered 😅

I just saw the original JavaScript's implement and converted it to TypeScript version.

https://the-algorithms.com/algorithm/merge-sort?lang=javascript

It truly comes at the cost of efficiency as you mentioned.

[benben6515]

BTW, I also have another version of quickSort.

Does it make sense?

function quickSort(array: number[]): number[] {
  if (array.length <= 1) return array.slice()

  const pivot = array[0]
  const lesser: number[] = []
  const greater: number[] = []

  for (let i = 1; i < array.length; i++) {
    if (array[i] < pivot) lesser.push(array[i])
    else greater.push(array[i])
  }

  return quickSort(lesser).concat(pivot).concat(quickSort(greater))
}

const arr = [3.14, 5, -2, 1, 10]
console.log(quickSort(arr))
// [ -2, 1, 3.14, 5, 10 ]

[appgurueu]

BTW, I also have another version of quickSort.

Does it make sense?

Yes, this is the "out of place" version which is easier to write but wastes memory. I don't think it should replace the in-place version. It might exist alongside it since it is more newbie-friendly.

[benben6515]

After rewriting, I finally got your point! I updated merge_sort.ts again ~

And here is not "out of place" version of quick_sort, but it is very similar to the in-place version already. 😁

function quickSort(array: number[], left: number = 0, right: number = array.length - 1) {
  if (left >= right) return
  const index = partition(array, left, right)
  quickSort(array, left, index - 1)
  quickSort(array, index + 1, right)
}

function partition(array: number[], left: number, right: number) {
  const pValue = array[right]
  let pIndex = left
  for (let i = left; i < right; i++) {
    if (array[i] < pValue) {
      const temp = array[i]
      array[i] = array[pIndex]
      array[pIndex] = temp
      pIndex++
    }
  }
  const temp = array[right]
  array[right] = array[pIndex]
  array[pIndex] = temp
  return pIndex
}

const arr = [3.14, 5, -2, 1, 10]
quickSort(arr)
console.log(arr)
// [ -2, 1, 3.14, 5, 10 ]

[appgurueu]

Can you get rid of the (unrelated?) changes to Quick Sort (or repurpose your PR accordingly)?

[benben6515]

Sure!, I converted the naming of quick sort 😁